(Bloomberg) -- President Joe Biden will promote the importance of U.S. and allied weapons for Ukraine’s defense on Tuesday with a visit to a Lockheed Martin Corp. plant making Javelin anti-tank missiles, which American and NATO officials say have mauled Russia’s invading armored columns. 

Biden’s trip to the Troy, Alabama, factory comes as the president is pushing lawmakers to authorize an additional $33 billion in aid for Ukraine -- and as lawmakers warn that stockpiles of the U.S. Army and Marine Corps’ premier anti-tank weapon are becoming depleted due to the war.

The self-guided portable missile system -- which weighs just 49 pounds (22 kilograms) -- has proved invaluable to Ukrainian fighters, who can fire on Russian vehicles from as far as 4,000 meters (2.5 miles) away and then quickly flee. The U.S. has already delivered about 5,000 of the 5,500 Javelins the White House has committed to provide to Ukraine, a senior defense official told reporters Monday. 

“To modernize Teddy Roosevelt’s famous advice: Sometimes we will speak softly and carry a large Javelin,” Biden said of the U.S. support late last month. 

U.S. and NATO officials have cited Javelins and other anti-armor systems as key to staving off a quick Russian victory in the early days of the war, buying time for President Volodymyr Zelenskiy’s government. Ukraine’s unexpected battlefield success defending its capital, Kyiv, forced Moscow to divert forces to the east and south and narrow its war aims. 

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The Lockheed factory in Alabama is one of three in the U.S. that produce Javelins, and 265 of the plant’s 600 employees support production of the anti-tank system. The facility can currently produce about 2,100 Javelins per year, the White House said.

The systems have become so synonymous with Ukrainian military success that a Canadian artist selling t-shirts and stickers of an Internet meme featuring the Virgin Mary clutching a Javelin has raised over $1 million for charity. 

“Russian armor is taking very high losses due to a combination of systems like Javelin as well as systems” delivered by drones, Air Force Secretary Frank Kendall said at a Brookings Institution event Monday. The Javelin’s arched missile flight allows targeting of an armored vehicle’s weakest points, resulting in a kill rate of over 90% during the Ukraine conflict, according to a White House official.

Still, some members of Congress have expressed alarm thatThe Lockheed factory in Alabama is one of three in the U.S. that produce Javelins, and 265 of the plant’s 600 employees support production of the anti-tank system. The facility can currently produce about 2,100 Javelins per year, the White House said. the diversion of Javelins -- as well as Stinger anti-aircraft missiles -- to Ukraine could leave the U.S. vulnerable, particularly as supply-chain issues make manufacturing replacements more challenging.

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Senator Richard Blumenthal, a Connecticut Democrat, said last month that the U.S. has already sent about a third of its Javelin systems to Ukraine, and that replenishing stocks could take more than two years.

Lawmakers and foreign policy experts have warned that the backlog could prevent the U.S. from providing crucial weapons systems to other allies facing the threat of invasion, like Taiwan. The weapon has been sold to 18 nations including Australia, Czech Republic, Estonia, France, Georgia, Indonesia, Ireland, Jordan, Lithuania, New Zealand, Norway, Oman, Qatar, Taiwan, Turkey, U.K. and United Arab Emirates.

“I think one of the observations that has been shared from the Pentagon and from a variety of quarters has been that we need our defense industry to be revitalized,” General David Petraeus, a former director of the Central Intelligence Agency, said Monday on Bloomberg’s “Balance of Power With David Westin.” The U.S. has shipped stockpiles “to Ukraine pretty quickly, and we do now need to replenish those and, again, to do that rapidly.”

Biden’s emergency request for Ukraine includes $16.4 billion for the Pentagon, on top of the more than $3.4 billion in military aid provided by the U.S. since the beginning of the invasion. While the majority of the request is designed to put weapons in the hands of Ukrainian forces, more than $5 billion is earmarked to replenish U.S. stocks.

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But even with that funding, the U.S. may struggle to replace key weapons systems. Raytheon Technologies Corp., which produces Stinger anti-aircraft missiles and partners with Lockheed on the Javelin, said in an earnings call last week that it would take years to fully replenish stockpiles.

Chief Executive Officer Greg Hayes said it wouldn’t be a matter of simply ratcheting up production, because the company has a “very limited stock” of material on hand and some components are no longer commercially available. The company is talking through the supplier constraints with the Pentagon, which hasn’t purchased Stingers in 18 years, Hayes said.

A senior defense official said the U.S. had prioritized transferring weapons that were closer to expiration when choosing what to send to Ukraine, and other administration officials downplayed the worries about replenishing supplies -- while emphasizing the need for more funding. 

White House Press Secretary Jen Psaki said the president was not concerned about the pace of Pentagon contracts to restock its supplies, and added that the Defense Department was working closely with industry officials to address potential bottlenecks.

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“We can work to address that in the manufacturing process and we’re considering a range of options, if they’re needed, to increase production capacity and improve production time lines of both Javelin as well as Stingers,” Psaki said.

Defense Department spokesman John Kirby declined to say how much the arms supplies for Ukraine have depleted U.S. stockpiles, but told reporters Monday that “we have not seen any negative impact on our ability to defend this nation across a range of military capabilities.”

Despite the supply worries, Ukraine is expected to continue to push hard for the weapons, which military commanders there have long desired. Lawmakers who traveled to Kyiv over the weekend as part of a congressional delegation led by House Speaker Nancy Pelosi said they discussed Ukraine’s need for additional weapons in meetings with Zelenskiy, and the country’s government previously said it hoped to receive 1,000 Stingers and Javelins per day as the conflict continued. 

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